A microfabrication technique is a very important basic technique in the field of semiconductor technology, and the research and development thereof have been progressed for further finer microfabrication. In recent years, particularly, with high integration of a large scale integration circuit, refinement of circuit patterns, wiring patterns, or contact-hole patterns for wiring between layers forming a cell has been progressed, and a request for microfabrication technology has been increased.
In connection with this, even in the field of technology for photomask production to be used in the process for photolithography in microfabrication, a technique for forming fine and correct circuit patterns (mask patterns) has begun to be demanded.
In order to high-precision processing by lithography technology, the precondition is to form a photomask pattern with high precision. Generally, reduction projection is performed when forming a pattern on a semiconductor substrate by photolithographic technique. The size of the pattern formed on the photomask is therefore approximately four times larger than the side of the pattern formed on the semiconductor substrate. However, this does not mean that the desired precision of the pattern formed on the photomask is smaller than the pattern formed on the semiconductor substrate. Rather, the precision of a pattern formed on the photomask as a master is desired to be higher more than an actual pattern obtained after exposure
In today's photolithography technical field, the size of a circuit pattern to be drawn is considerably smaller than the wavelength of light to be used for exposure. Thus, in the case of forming a photomask pattern with a just four-times larger circuit pattern, light interference or the like, which is generated under exposure, influences on transfer of an original shape. As a result, the original shape cannot be transferred onto the photoresist film of a semiconductor substrate.
In some cases, therefore, a pattern formed on the photomask is made more complicated than an actual circuit pattern to reduce an effect of the above light interference or the like. The shape of such a pattern may be, for example, an actual circuit pattern subjected to optical proximity correction (OPC).
Hence, along with a decrease in size of a circuit pattern, a higher precision processing technique has been also desired in a lithographic technique for forming photomask patterns. Although lithography performance may be expressed in limiting resolution, as described above, the precision of a pattern formed on the photomask as a master is desired to be higher more than an actual pattern obtained after exposure. Thus, limiting resolution required for formation of a photomask pattern is almost equal to or higher than one required in lithography for forming a pattern on a semiconductor base.
In general, when forming a photomask pattern, a photoresist film is formed on the surface of the photomask blank in which a light-shielding film is mounted on a transparent substrate, and a pattern is then drawn (exposed) on the photoresist film by an electron beam. Subsequently, after obtaining a photoresist pattern after developing the exposed photoresist film, the light-shielding film is etched by using this photoresist pattern as a mask to obtain a light-shielding (film) pattern. The light-shielding (film) pattern thus obtained is served as a photomask pattern.
In this case, the above photoresist film should be thinned depending on the degree of fineness of the light-shielding pattern. This is because, when forming a fine light-shielding pattern while keeping the thickness of the photoresist film, the ratio (aspect ratio) of the thickness of the photoresist film to the size of the light-shielding pattern becomes large and causes troubles of failed pattern transfer, falling down or peeling off of the photoresist pattern, or the like due to deterioration of the shape of the photoresist pattern.
As a material of the light-shielding film mounted on the transparent substrate, many kinds of materials have so far been proposed. Among them, however, a chromium compound has been practically used because of much know-how on etching, for example.
Dry etching of a chromium-containing material film is generally performed by chlorine-containing dry etching. In many cases, however, chlorine-containing dry etching has a certain level of ability to etch an organic layer. Thus, in the case that a photoresist pattern is formed on a thin photoresist film and then used as a mask to etch a light-shielding film, the photoresist pattern is also etched too much to ignore by chlorine-containing dry etching. As a result, the proper photoresist pattern, which should be transferred to a light-shield film, cannot be correctly transferred to the light-shielding film.
In order to avoid such inconvenience, a photoresist material having excellent etching resistance has been requested. However, such a photoresist material has not been known yet. For this reason, to obtain a light-shielding (film) pattern having high resolution property, a light-shielding film material having higher processing accuracy is required.
For a light-shielding film having higher processing accuracy as compared with a conventional material an attempt to increase the etching rate of a light-shielding film is reported, the attempt allowing a chromium compound to contain only a certain amount of a light element.
For example, Patent Literature 1 (WO 2007/74806 A) discloses a technique for reducing photoresist film loss at the time of chlorine-containing dry etching by using a material mainly containing chromium (Cr) and nitrogen (N) and having an X-diffraction peak of substantially CrN(200) as a light-shielding film material to enhance a dry-etching rate.
Furthermore, Patent Literature 2 (JP 2007-33470 A) discloses the invention of a photomask blank where its composition, film thickness, and a laminated structure are suitably designed to obtain desired transmittance T and reflectance R while trying to increases a dry-etching rate by making the composition of a chromium-containing compound of the light-shielding film rich in light element and low in chromium composition as compared with the composition of the conventional film.